1. Field of the Invention
The present invention relates to an X-ray mask used in a stepper for transferring microscopic patterns onto a semiconductor substrate. More particularly, this invention relates to an X-ray mask, wherein deformation of mask patterns due to stresses in the X-ray absorbing material is effectively suppressed.
2. Description of the Related Art
Photo-lithography technology is widely used for forming patterns on semiconductor wafers. Using a stepper, the patterns formed on a mask are transferred optically onto a resist layer of a wafer by a step-and-repeat method, in which the semiconductor wafer is moved in X and Y directions and is exposed at specified positions on the wafer.
The most common prior art approach in photo-lithography has used ultra-violet light to expose the patterns on the wafer. However the ultra-violet light technique has resolution limitations. Diffraction, interference, or light scattering is common, causing reduction of resolution. With the trend toward a higher integration of semiconductor devices, the need to form patterns having up to submicron geometries has developed. This need has been filled in part by X-ray lithography.
In the X-ray lithography, an X-ray source is used to direct X-rays through a mask and onto a resist layer of a semiconductor wafer. The wavelength of X-rays is very short and ranges generally from a few Angstroms to 20 Angstroms. Therefore, conventional mask structures as used with photo-lithography cannot be used. Instead, X-ray masks having a special structure are necessary. Generally, the processes used in X-ray lithography are very similar to those used in photo-lithography. However, X-ray lithography and mask technology have their own problems, which are outlined in "X-ray Lithography and Mask Technology" by Pieter Burggraaf, Semiconductor International, April, 1985, pp. 92-99.
A semiconductor wafer to be processed is mounted on a stage. The X-ray source irradiates an X-ray resist layer on the semiconductor wafer surface through the X-ray mask, thereby transferring the patterns on the X-ray mask to the X-ray resist layer of the wafer. The stage is then moved in X and Y directions to a new predetermined position, and a new location on the wafer is then exposed. This process, conventionally called the "step-and-repeat" method, is repeated a plurality of times. The apparatus used in the step-and-repeat method is called a stepper.
FIG. 1 shows a plan view of a typical X-ray mask 10, and FIG. 2 shows a schematic cross sectional view thereof. A ring 11 of pyrex glass or ceramic material supports a silicon ring 12, which supports a membrane 13. In the X-ray mask making process, originally the silicon ring 12 is a silicon wafer, on which the membrane 13 is formed by growing a layer of inorganic material such as boron nitride (BN), silicon carbide (SiC), or silicon nitride (SiN). This is performed using a Chemical Vapor Deposition (CVD) method, with a resulting thickness of membrane 13 being about 1 to 5 microns. Properties required of the membrane 13 are that it not incur deterioration of mechanical strength due to X-ray irradiation and that it remains flat after the formation of mask patterns thereon.
After formation of the membrane 13 on the silicon wafer, most of the silicon wafer is etched away, leaving the silicon ring 12. The periphery of the membrane 13 is fixed by the silicon ring 12, imparting a controlled tension in the membrane 13 to maintain its flatness.
A layer 25 of X-ray absorbing material, such as gold, is deposited on the membrane 13. The process for depositing the layer 25 is described later. In FIG. 1, a single chip pattern comprising mask patterns 14 comprised of the X-ray absorbing material is illustrated in an effective pattern region 15. The effective region 15 is surrounded by a covered region 16 of the layer 25. In FIG. 1, the areas of the layer 25 on the element 13 which remain after the formation of mask patterns 14 are represented by the dotted areas.
FIG. 3 schematically illustrates the X-ray mask 10 installed in a stepper 30. In exposing a portion of a silicon wafer, an X-ray source 31 generates an X-ray beam 32 which passes through an aperture 33. The X-ray beam 32 then passes through the mask 10 and onto a silicon wafer (not shown) having an X-ray resist layer. The mask patterns 14 of the effective pattern region 15 of the X-ray mask 10 is thereby transferred to the X-ray resist layer on the silicon wafer. The stage on which this silicon wafer is mounted is then moved, and the exposure step is then repeated for another portion of the wafer. This process is repeated a predetermined number of times.
FIGS. 4(a) to 4(e) are schematic cross sectional views of the effective pattern region 15 illustrating the formation of mask patterns on the X-ray mask.
In FIG. 4(a), a layer 21 of electrically conductive material composed of, e.g., tantalum/gold/tantalum (Ta/Au/Ta), or gold/titanium (Au/Ti), is formed on the membrane 13. The conductive layer 21 has a thickness of 300 to 500 Angstroms, and functions as a plating base for the subsequent step. A polyimide layer 22 having a thickness of 0.5 to 2 microns is formed on the conductive layer 21, and a second inorganic material layer 23, composed of, e.g., silicon dioxide (SiO.sub.2), having a thickness of 500 to 2000 Angstroms is formed on the polyimide layer 22. Finally, an electron beam (EB) resist layer 24 of 1000 to 5000 Angstroms thickness is coated on the second inorganic material layer 23.
The EB resist layer 24 is exposed to an electron beam and is patterned as shown in FIG. 4(b). The exposed inorganic layer 23 is then etched away by a Reactive Ion Etching (RIE) method using a mixed gas of trifluoromethane (CHF.sub.3) and terafluoride (CF.sub.4). The exposed polyimide layer 22 and remaining EB resist layer 24 are etched away by an RIE method using oxygen (O.sub.2) gas, leaving a so-called stencil structure 27 which forms the pattern, as shown in FIG. 4(c).
As illustrated in FIG. 4(d), the conductive layer 21 is electrically gold plated, filling the apertures of the stencil structure 27 with gold. The gold functions as the X-ray absorbing layer 25. The remaining inorganic layer 23 and polyimide layer 22 of the stencil structure 27 are then removed by an RIE method using the same gases as those mentioned previously, and the exposed electrically conductive layer 21 is etched away. The removal of electrically conductive layer 21 is necessary for making portions of the X-ray mask translucent to light beams during an alignment procedure for the X-ray mask, which uses alignment marks on the silicon wafer. Finally, as shown in FIG. 4(e), a protection layer 26 is coated over the entire mask. This completes the process of forming patterns on the X-ray mask 10, and FIG. 4(e) shows the finished surface of the X-ray mask 10.
The above method of forming patterns on the X-ray mask is one of several methods that can be utilized. For example, other X-ray absorbing materials, such as tungsten (W) or tantalum (Ta), can be sputtered or coated in other ways on the membrane without using an electrically conductive layer. These materials are subsequently etched to form the patterns.
The X-ray mask must have precise patterns situated on a flat surface of the membrane. Deformations or deviations from the designed dimensions of the patterns must be avoided as much as possible. In order to have the strict properties of flatness and stability required during the pattern forming processes and use in the stepper, the membrane has a thickness of 1 to 5 microns. The exact membrane thickness is determined by the material used for the membrane; for example, when boron nitride is used the preferred membrane thickness is 4 to 5 microns; and when silicon carbide or silicon nitride is the material used the preferred thickness is 2 microns. The thickness depends on a transmission factor for X-ray beams through the membrane material and the material properties for providing tension adequate to maintain the required flatness; therefore, the exact membrane thickness is determined experimentally for the specific material to be used.
Furthermore, the stress generated within the patterned absorbing material becomes an important factor in the design and manufacture of X-ray masks. The tensile or compressive stress in the absorbing material causes displacements of the fringe portions of the formed patterns.
FIGS. 5(a) and 5(b) illustrate a simple example of the stress problem. The absorbing material layer 25 having a simple band-shaped pattern is formed on the memberane 13. FIG. 5(a ) shows the ideal situation, in which the absorbing material layer 25 has zero stress. The band of absorbing material 25 has a length of D and maintains a flat surface. However, in practice, the absorbing material 25 will be subject to tensile or compressive stress. In the case when tensile stress exists, the band of absorbing material 25 shrinks to a length of D', and the membrane 13 and the absorbing material layer 25 are slightly deformed to an upwardly concave configuration, such as is shown in FIG. 5(b). Conversely, if compressive stress exists, the absorbing material 25 would elongate somewhat.
FIG. 6(a) shows the desired situation, where zero stress exists in the absorbing material 25. In the heretofore known practice, however, this result has been difficult to obtain. FIG. 6(b) shows the X-ray mask 10 having the absorbing material layer 25 patterned in the effective pattern region 15 and the non-patterned absorbing material layer 25 in the covered region 16. In FIG. 6(b), the absorbing material layer 25 in both the effective pattern region 15 and the covered region 16 have shrinkage stress, which deforms the surface of the mask 10 to a slightly upwardly convex configuration. The upward deformation in FIG. 6(b) is somewhat exaggerated, however, the displacement of the absorbing material layer 25 and therefore the displacement of the desired pattern along the surface of the mask 10 cannot be ignored. When this X-ray mask 10 is used in the lithography process, the precise pattern will not be reproduced in the resist layer of the silicon wafer.
In the X-ray masks of the prior art, the absorbing material layer 25 is formed without the use of an aperture, and thus the layer 25 forms on the entire surface of the covered region 16, with no consideration given to the absorbing material stress and the resultant displacement of the pattern.
Serious effort has been devoted to minimize the deformation of the mask patterns; however, up to now deformation has remained a difficult problem not yet solved.